This section introduces aspects that may help facilitate a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
For certain optical network applications, it is desirable to lock multiple optical wavelengths to particular different channels of a grid, such as the dense wavelength division multiplexing (DWDM) standard grid channels of the International Telecommunication Union (ITU). The transmission of dense multiplexed optical carriers with carrier separations of, e.g., 50 or 25 GHz, should be closely locked to their respective dedicated optical wavelength to avoid inter-channel crosstalk.
Sometimes, wavelength locking is achieved using a reference Fabry-Perot etalon along with complex temperature measurement and feedback adjustment circuitry to achieve thermo-optic tuning. Sometimes, the etalon is made of quartz glass due to its small thermo-optic coefficient and linear thermal expansion coefficient. The construction of such a reference etalon on silicon photonic integrated circuits (PICs) can be problematic, however, due to the large thermo-optic coefficients of silicon which makes the temperature stabilization problematic. The integration of quartz glass Fabry-Perot etalons on a PIC also may not be attractive because of the relatively large dimensions of such etalons.
Attempts to provide laser locking using silicon interferometer or ring filter designs are subject to fabrication variations and may require aggressive active thermo-optic tuning procedures due to the large thermo-optic coefficient of silicon. The use of passive athermal filter designs may provide a means to eliminate or reduce such active stabilization requirements. For instance, certain interferometer filter designs, where the two arms of the interferometer have different waveguide widths, have been shown to exhibit athermal behavior in a particular wavelength range.